Sulfated Tin Oxide as Highly Selective Catalyst for the Chlorination of Methane to Methyl Chloride

Article abstract of DOI:10.1021/acscatal.9b02645

The chlorination of methane (CH₄) is an attractive route to convert CH₄ into more valuable chemicals. The selective formation of methyl chloride (CH₃Cl) is a key process, but it is rather difficult to achieve with high selectivity due to a radical reaction. Catalytic ionic processes can be a solution. Herein, sulfated tin oxide (STO) was employed in the gas-phase catalytic chlorination of CH₄. The STO catalyst exhibited high selectivity to CH₃Cl (>96%) even at high CH₄ conversion. By applying a suite of physicochemical characterizations, it is shown that the strong Lewis acid sites on STO generated by the interaction of Sn and surface sulfate groups are mainly responsible for the highly selective CH₄ conversion. DFT calculations further revealed that STO surface can activate more Cl₂ molecules in a heterolytic manner, leading to better catalytic performances as compared to SnO₂ and sulfated zirconia catalysts.

Full text of DOI:10.1021/acscatal.9b02645

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Article
Sulfated Tin Oxide as Highly Selective Catalyst
for the Chlorination of Methane to Methyl Chloride
Youngmin Kim, Jip Kim, Hyun Woo Kim, Tae-Wan Kim, Hyung
Ju Kim, Hyunju Chang, Min Bum Park, and Ho-Jeong Chae
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02645 • Publication Date (Web): 06 Sep 2019
Downloaded from pubs.acs.org on September 6, 2019
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Sulfated Tin Oxide as Highly Selective Catalyst for
the Chlorination of Methane to Methyl Chloride
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†,1
†,1,2
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Youngmin Kim , Jip Kim , Hyun Woo Kim , Tae-Wan Kim , Hyung Ju Kim , Hyunju
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*,1,4
Chang , Min Bum Park , and Ho-Jeong Chae
1Carbon Resources Institute, Korea Research Institute of Chemical Technology, Daejeon 34114,
Republic of Korea
2Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 04107,
Republic of Korea
3Center for Molecular Modeling and Simulation, Korea Research Institute of Chemical
Technology, Daejeon 34114, Republic of Korea
4Department of Green Chemistry & Biotechnology, University of Science and Technology,
Daejeon 34113, Republic of Korea
5Department of Energy and Chemical Engineering, Incheon National University, Incheon 22012,
Republic of Korea.
†These two authors contributed equally to this work.
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ABSTRACT: The chlorination of methane (CH ) is an attractive route to convert CH into more
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valuable chemicals. The selective formation of methyl chloride (CH Cl) is a key process, but it is
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rather difficult to achieve with high selectivity due to a radical reaction. Catalytic ionic processes
can be a solution. Herein, sulfated tin oxide (STO) was employed in the gas-phase catalytic
chlorination of CH . The STO catalyst exhibited high selectivity to CH Cl (>96%) even at high
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CH conversion. By applying a suite of physicochemical characterizations, it is shown that the
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strong Lewis acid sites on STO generated by the interaction of Sn and surface sulfate groups are
mainly responsible for the highly selective CH conversion. DFT calculations further revealed
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that STO surface can activate more Cl molecules in a heterolytic manner, leading to better
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catalytic performances compared to SnO and sulfated zirconia catalysts.
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KEYWORDS: methane, chlorination, methyl chloride, sulfated tin oxide, heterogeneous
catalysis, selectivity
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. INTRODUCTION
Methane (CH ), the main component of natural gas, is an important feedstock for the synthesis
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of fuels and value-added chemicals, such as methanol, light olefins, and aromatics.1-4 With
ongoing discovery of large reserves of shale gas, the utilization of methane has received
increased attention in recent years. At present, industrially, methane is converted into bulk
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chemicals via an indirect process. That is, methane is reformed to syngas (CH + H O → CO +
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H ) first, and then the syngas is converted into higher hydrocarbons or oxygenates using the
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Fischer-Tropsch (FT) process.5,6 The other routes includes sequential transformation of methane
to syngas, syngas to methanol, and then methanol to olefins (MTO), methanol to gasoline
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(MTG), or methanol to aromatics (MTA). However, these processes are complex, energy
intensive, and the capital cost is high. Therefore, the direct conversion of methane to value-added
products under mild conditions with much less energy has been a subject of great importance in
economical methane utilization.
Many efforts have been given to the direct conversion of methane to the desirable products in
high yields. Methane is an inert molecule that has perfectly symmetric structure. It is known that
the activation of first C-H bond of methane is difficult because of very high dissociation energy
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(ca. 440 kJ mol ) and low polarity of C-H bond. High temperature, pressure, and/or oxidizing
agents are needed for the direct activation of methane, and at such conditions gas phase free-
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radical reactions dominantly occur, which results in lack of selectivity. For the selective
conversion of methane, catalysis will have to play a key role. Several different approaches for
the direct conversion pathway of methane to chemicals have been proposed like thermal or
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catalytic pyrolysis , oxidative or nonoxidative coupling , partial oxidation , plasma process ,
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halogenation , photo-catalysis
, and membranes etc. Among them CH halogenation is one
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of the more efficient process for the direct transformation of CH . This reaction can produce
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mono-methyl halides (CH X, where X is halogen), which can act as reaction intermediates like
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methanol (CH OH) to further produce higher hydrocarbons or oxygenates.
Since the
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reaction of methane with fluorine is too reactive to control the reaction (highly exothermic) and
the reaction with iodine is too stable to proceed the reaction (endothermic), most of the research
on the methane halogenation have focused on the use of chlorine or bromine as a halogen source.
Among the halogens, chlorine (Cl ) is the most widely used element in worldwide industry,
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being largely applied as a building block in the manufacture of a variety of polymers (polyvinyl
chloride (PVC), polyurethane (PU), and polycarbonate (PC)), drugs, and agricultural chemicals
19. The use of chlorine in methane halogenation is beneficial for large-scale applications because
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of its proper reactivity, high availability, and low price. Also, excess chlorine, which is harmful
to the environment and human health, can also be effectively consumed via the CH chlorination
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process. In this process, CH reacts with Cl to produce chlorinated mixtures of methyl chloride
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CH Cl), methylene chloride (CH Cl ), chloroform (CHCl ), and carbon tetrachloride (CCl )
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with equimolar hydrogen chloride (HCl) (Eqs. 1-4).21
CH + Cl → CH Cl + HCl
(1)
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CH + 2Cl → CH Cl + 2HCl
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CH + 3Cl → CHCl + 3HCl
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CH + 4Cl → CCl + 4HCl
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Industrially, CH Cl is an important product as a valuable starting material in the production of
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higher chlorinated products, silicones, and methyl cellulose. It also can be potentially used for
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the production of olefins, gasoline, or aromatics. While, CH Cl , CHCl , and CCl are un-
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desired products due to their harmful effects contributing to global warming, the depletion of the
ozon layer, and the formation of photochemical smog.24,25 Hence, CH Cl is the most desired
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product of the methane chlorination reaction. However, gas-phase CH chlorination follows a
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free-radical chain mechanism yielding low selectivity toward the desired CH Cl, especially at
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high CH conversion.
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Heterogeneous catalysts can facilitate the selective mono-chlorination of methane to produce
methyl chloride under the proper reaction conditions. A few studies have been devoted to
methane chlorination using heterogeneous catalysts. Heterogeneous catalytic gas-phase CH4
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chlorination was first reported in the mid 1980’s by Olah et al. They demonstrated that CH
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could be selectively chlorinated to CH Cl over supported metal oxyhalides and Pt (or Pd)
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catalysts at relatively low temperature, but also at very low space velocity. They proposed that
the process involves an ionic mechanism through electrophilic insertion reactions. Cl is
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polarized at the Lewis acid sites of the catalyst and the Cl species complexed to the catalyst
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surface activate the C-H bonds of CH via formation of intermediate five-coordinated carbonium
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ions, and then subsequent halogenolysis to CH Cl. Complexing ability of the catalyst with
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chlorine and polarizability of chlorine on the catalyst surface are key factors in this mechanism.
Later, several zeolites such as H-ZSM-5, H-mordenite, NaL, X, and Y were examined as acid
catalysts for CH chlorination, and the results showed that zeolites having higher acidity led to
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better selectivity toward CH Cl. The zeolite catalysts, however, were rapidly deactivated in a
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short time due to the dealumination of the zeolite matrix by HCl. Sulfated zirconia (SZ), one of
the strongest solid super acids, was also studied as a CH chlorination catalyst to produce CH Cl
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with >90% selectivity, albeit at a CH conversion of less than 10%. It was observed that the
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selectivity decreased to ~70% when the methane conversion increased up to 24% over the SZ
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catalyst. Very recently, Na et al. has investigated the simultaneous increase in CH conversion
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and CH Cl selectivity using a Pt loaded NaY zeolite catalyst with a Frustrated Lewis Pairs
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system, but it still yielded a low CH Cl selectivity of ~60% at CH conversion of ~35%.
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Sulfated tin oxide (STO) is known to possess strong acidic properties and belongs to the class
of solid super acids. In previous reports, it was proved that the acid strength of STO is even
higher than that of SZ.30-32 The generation of strong Lewis acid sites by sulfate species is due to
the presence of a covalent S=O bond, which acts as electron-withdrawing species followed by
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the inductive effect, making the Lewis acid strength of Sn stronger. Although STO has been
used as a catalyst in various acid catalyzed reactions such as etherification , esterification33,34,
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Friedel–Crafts acylation , alkylation , transesterification
, dehydration , and epoxidation
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reactions , there has been no literature concerning the use of STO as a catalyst for CH
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chlorination. Thus, in this work, STO was synthesized and investigated as a solid super acid
catalyst in gas-phase CH chlorination for the selective production of CH Cl. Un-sulfated pure
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tin oxide (SnO ) and SZ catalysts were also prepared and tested for comparison. The STO
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catalyst could activate Cl and sequentially CH to produce CH Cl with consistently high
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selectivity even at high CH conversion. The effects of sulfate amount, calcination temperature,
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methane to chlorine ratio, and gas hourly space velocity (GHSV) were also studied. The possible
reason for the high selectivity to CH Cl over STO was discussed based on the calculations of
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density functional theory (DFT).
2. EXPERIMENTAL SECTION
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.1. Catalyst Synthesis. Under magnetic stirring, SnCl ·5H O (100 g, 98%, Sigma-Aldrich)
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was dissolved in deionized water (2 L), and aqueous ammonia (28-30%, Sigma-Aldrich) was
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dropwise added until the solution was adjusted to pH 8. The solution was continuously stirred
overnight, and the precipitated product was washed with deionized water and collected by
centrifugation. Sn(OH) powder was obtained after drying at 100 °C for 12 h and grinding. The
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sulfation was performed by adding Sn(OH) powder into the 3 M H SO (95-98%, Sigma-
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Aldrich) aqueous solution (15 ml g ) and vortex mixed vigorously for 30 min. The sulfated
hydroxide material was collected by centrifugation, dried at 100 °C for 12 h, and finally calcined
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at 500 °C (3.3 °C min ) in air for 3 h to form STO. For comparison, pure SnO was prepared by
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the same procedure without the sulfating process. SZ was also prepared and tested as another
reference catalyst. Zr(OH) (97%, Sigma-Aldrich) was used as a precursor, and it was sulfated
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with 3 M H SO aqueous solution and calcined at 500 °C (3.3 °C min ) in air for 3 h before use.
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In order to investigate the effect of catalyst preparation conditions on the methane chlorination
reaction, we also prepared some more STO catalysts: i) sulfated with different concentrations of
H SO aqueous solution and calcined at the same 500 °C; ii) sulfated with the same 3 M H SO
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but calcined at different temperatures. The catalysts were designated STO-xM H SO (x = 1, 2,
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or 3) and STO-y °C (y = 400, 500, or 600), respectively. The other conditions were identical to
those given above.
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.2. Catalyst Characterization. The crystallographic structures of the catalysts were
confirmed by X-ray diffraction (XRD) (Rigaku Ultima IV) operating at 40 kV and 40 mA with a
Ni-filtered Cu Kα (λ = 1.5418 Å) source. Fourier-transform infrared (FT-IR) spectra in the 400-
-1
4000 cm wavenumber range were recorded on a Bruker ALPHA-T FT-IR spectrometer. The
specific surface areas and pore sizes were calculated by the Brunauer-Emmett-Teller (BET) and
Barrett-Joyner-Halenda (BJH) methods, respectively, by using N adsorption-desorption at 77 K
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with a Micrometrics ASAP 2020 analyzer. The crystal morphologies were examined via
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transmission electron microscope (TEM) (Tecnai G2 F30 S-Twin) operated at 300 kV. During
the TEM measurement, elemental analysis was also carried out using an attached energy
dispersive X-ray (EDX) spectrometer. The acid properties of the catalyst were studied by using
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temperature-programmed desorption of ammonia (NH -TPD) using a BELCAT-B analyzer
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equipped with a thermal conductivity detector (TCD). Temperature-programmed reduction
(
TPR) with CH was also carried out on a same analyzer. The relative densities of Brønsted and
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Lewis acid sites were estimated using the pyridine FT-IR (Nicolet 6700) technique with self-
supporting catalyst wafers. X-ray absorption near edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) spectra were collected on the 7D beamline at Pohang
Acceleration Laboratory (PLS-II, 3.0 GeV, Korea). Thermogravimetric analyses (TGA) were
performed on a Thermo plus EVO II TG8120 series, where the weight losses were related to the
combustion of coke contents. Those were further confirmed by using a Thermo Scientific Flash
2000 CHNS elemental analyzer (EA).
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.3. Reaction testing. CH chlorination as well as CH Cl chlorination was carried out in an
4 3
Inconel fixed-bed reactor (inner diameter = 7 mm, length = 460 mm) under atmospheric pressure
at 300 - 350 °C. 0.5 g of the catalyst was loaded inside the fixed-bed reactor. The gas flows of
CH (99.999%, Rigas Korea), CH Cl (99.5%, Rigas Korea), Cl (99.999%, Rigas Korea), and He
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(
99.9999%, Daesung Gas) were adjusted using mass flow controllers (Line-Tech for CH and He,
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Brooks for CH Cl and Cl ). The stainless steel gas lines of the setup were heated to 150 °C to
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prevent product condensation and corrosion. In order to prevent Cl and CH (or CH Cl) from
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reacting in advance in the reactor voids, a Cl gas line was installed to inject Cl gas directly
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above the catalyst bed. Prior to the reaction, the catalyst was pretreated at 400 °C for 1 h in a
-1
flow of He (10 mL min ). The reactor was then set at the reaction temperature and fed with CH
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(
or CH Cl) Cl , and He. In a typical reaction, CH chlorination sequentially proceeded with an
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increasing mode for 2 h at each temperature (300, 325, and 350 °C), and thus the total reaction
time was around 6 h. To investigate the stability of the catalyst, the independent isothermal
reactions were also performed at each temperature during about 6 h on stream. The products
were analyzed online by a gas chromatography (GC) (Younglin GC-6100 series) equipped with a
HP-PLOT Q column (30 m length, 0.53 mm diameter, 40 μm thickness) and a flammable
ionization detector. After analysis, the gas exhausts were passed through a NaOH scrubber to
remove HCl and unreacted Cl . The CH conversion and product selectivity were calculated on a
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carbon mole basis and defined using the following equations.
Conversion (%) = [{carbon (mole) in initial CH (or CH Cl) – carbon (mole) in CH (or CH Cl)
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after reaction} / {carbon (mole) in initial CH (or CH Cl)}] × 100
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Selectivity (%) = [{carbon (mol) in a product} / {carbon (mol) in all product}] × 100
In the all reactions, the product mixture contained only CH Cl, CH Cl , CHCl , and unreacted
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CH , and no other organic species were detected (Supporting Information Figure S1). In
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addition, no evidence of coke content of the spent catalysts was confirmed by TGA (Figure S2)
and EA (Table S1) analyses. Therefore, we simply calculated the carbon balance from the sum of
CH , CH Cl, CH Cl , and CHCl , and the resulting carbon balance was confirmed to be ca.
4
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00% under all the reaction conditions studied here (Table S2).
2
.4. Computational Details. All DFT calculations were performed as implemented in the
Vienna ab initio simulation package (VASP).41,42 We employed the optB86b-vdW functional43,44
to account for van der Waals interactions between molecules and catalytic surfaces. The
pseudopotentials of all atoms were described using the projector augmented wave (PAW)
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method. We sampled the Brillouin zone with 3×3×1 Monkhorst-pack k-point grids. The kinetic
energy cutoff was set to 450 and 520 eV for structures containing Sn and Zr, respectively. The
ZrO (110) and SnO (110) surfaces were modeled as 4×4 supercells using the bulk structure
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deposited at the Materials project as explained in a previous report. A total of four layers
were included in our model and two upper layers were allowed to relax, while the two other
layers were fixed during all calculations. Sulfated structures were generated by adding SO4
moiety to two metal atoms exposed to the surface. Periodic boundary conditions were applied to
all surface structures which were separated along the c-axis by a vacuum distance of 20 Å. All
geometries were optimized until the force was less than 0.01 eV/Å. We computed the adsorption
energies of Cl and CH Cl by referencing the gas phase energies of the surfaces and adsorbed
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molecules. In our sign convention, energetically favorable or exothermic adsorptions show
negative adsorption energies. In explaining the catalytic activity toward the electrophilic
chlorination, we performed DFT calculations on catalytic surfaces with and without Cl atoms.
For comparing the relative stability of surfaces, we ignored the spin polarization. Atomic charges
were determined based on the Bader charge analysis.47,48 In the case of STO, the C-H bond
activation was investigated in a cluster model containing 27 Sn atoms in which the dangling
bonds are properly terminated by hydrogen atoms. As a H atom of CH or CH Cl approaches to
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Cl atom during the chlorination, the activation energy for the C-H bond can be estimated by
scanning the H-Cl distance up to 1.4 Å. To understand the polarization effect of the Cl atom, we
computed the C-H activation energies for a small model system including a CH molecule and a
4
Cl atom by adding a fixed amount of charge to the Cl atom. These computational modelings on
C-H activation processes were performed at M06/6-31G(d,p) and LANL2DZ (for Sn atoms)
level of theory with Q-Chem software package.49
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3
. RESULTS AND DISCUSSION
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.1. Physicochemical Properties of Synthesized Catalysts. Figure 1a shows the powder
XRD patterns of the three representative catalysts (i.e., SnO , STO, and SZ) prepared in this
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study. Both the SnO and STO samples show obvious X-ray peaks at 2θ = 27, 34, 38, 52, and
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5°, which are assigned to the tetragonal phase of the rutile SnO structure with P42/mnm
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symmetry. On the other hand, the X-ray peaks for STO become much broader and the intensity
is significantly decreased compared to those of un-sulfated SnO . This indicates a reduction in
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crystal size after sulfation, which was also reported in the literature. The XRD pattern of SZ
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also shows five obvious peaks assigned to the tetragonal phase of ZrO . The crystallite sizes of
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the three catalysts estimated from the Scherrer equation and the (110) and (101) X-ray peaks of
SnO and ZrO (Figure 1a), respectively, were 10.1, 1.8, and 7.1 nm. The BET surface areas of
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the sulfated samples (i.e., 138 and 126 m g for SZ and STO, respectively) were much higher
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than that of the un-sulfated SnO (25 m g ) (Table 1) known that the smaller the crystal size, the
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higher the surface area.
Figure 1b shows the FT-IR spectra of the SnO , STO, and SZ catalysts. For all three catalysts,
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the IR bands around 3400 and 1630 cm are due to the presence of surface hydroxyl groups and
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adsorbed water molecules, respectively. The characteristic peaks in the region of 610-500 cm
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are associated with the bending vibration of the Sn-O-Sn or Zr-O-Zr bonds. Characteristic
-1
sulfate bands are generally observed in the range of 1000-1200 cm , assigned to inorganic
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chelating bidentate sulfate ions coordinated to the metal cation. Both STO and SZ show several
bands around that region, while they cannot be observed in pure SnO . This confirms the
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presence of sulfate groups (SO ) on the surface of STO and SZ. We also note here that the
4
intensity of the IR bands for SO42- was much higher for SZ than STO, indicating more sulfate
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groups were adsorbed on the SZ surface. This is well correlated with the elemental analysis, i.e.,
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.6 and 3.0 wt% of sulfur in STO and SZ, respectively (Table 1).
Figure 2 shows the TEM and high resolution (HR) TEM images of the SnO , STO, and SZ
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catalysts. All the samples seem to have an agglomeration of irregular particles with small voids.
From their HRTEM images, the average crystal sizes of SnO , STO, and SZ were estimated to be
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ca. 11, 4, and 7 nm, respectively (Table 1), which is in good agreement with the XRD analysis.
Clear lattice fringes were also observed in all the samples with d-spacings of 0.33 and 0.29 nm,
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which can be indexed to the (110) SnO and (111) ZrO crystal planes, respectively.
To
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investigate the distribution of elements included in the catalysts, scanning TEM-EDX mapping
was also performed for STO and SZ catalysts (Figure 3). The sulfur species of the sulfate groups
were homogeneously distributed over the entire metal oxide surfaces of both catalysts.
Figure 4a displays the NH -TPD profiles for the SnO , STO, and SZ catalysts. The strength
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and distribution of acid sites calculated from the NH desorption peak area are also listed in
3
Table 1. The peaks in the range below 200 °C, 200-400 °C, and above 400 °C are normally
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attributed to weak, medium, and strong acid sites, respectively. Obviously, STO has the largest
acid site density, while SnO has almost no acid sites. This is in good agreement with the
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previous report. SZ has a broad peak below 400 °C and a relatively sharp peak around 520 °C.
For STO, there are two intense peaks observed around 180 and 530 °C, and the areas are much
larger than those for SZ, indicating that it has many more acid sites both with weak and strong
acid strength.
To distinguish the Brønsted and Lewis acid sites, FT-IR spectra of the three catalysts
chemisorbed pyridine were also obtained (Figure 4b). The IR bands around 1540 and 1450 cm-1
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are due to the adsorbed pyridine on Brønsted and Lewis acid sites, respectively. In addition,
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another intense band which appeared around 1490 cm corresponds to the combination of Lewis
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and Brønsted acid sites. SnO shows almost no pyridine adsorption IR bands, while the bands
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for both the Lewis and Brønsted acid sites were significantly increased by sulfation (i.e., IR
spectrum of STO). This is well correlated with the results from NH -TPD. The relative area ratio
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of Brønsted and Lewis acid sites (B/L) was calculated to be 2.5 and 7.8 for STO and SZ,
respectively (Table 1). This indicates that both catalysts have absolutely more Brønsted acid
sites, but the STO contains relatively more Lewis acid sites in comparison to SZ. The presence of
stronger Lewis acid sites in STO compared to SZ is also indicated by the band shift to a high
-1 33,59
energy wavenumber region (1454 vs 1446 cm ).
The STO catalysts prepared with different
o
sulfate concentration (STO-2M vs STO-3M) and calcination temperature (STO-400 C vs STO-
o
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00 C) were also compared for the pyridine-IR spectroscopy (Figure S3). As can be easily
expected, the amount of acid sites for STO-2M was lower than that of STO-3M despite the
similar B/L ratio (2.5). However, when decreasing the calcination temperature, the strength of
Lewis acid was weaker while the amount of Lewis acid sites increased.
Figure 5a shows Sn K-edge XANES spectra for the SnO and STO catalysts. The absorption
2
edge structure of STO is similar to that of SnO . However, there is a slight shift to a higher
2
energy in the edge position for the STO (29204.6 vs 29201.6 eV), indicating the oxidation state
is increased due to its higher Lewis acid site density.60,61 Figure 5b shows Fourier transforms
3
(FT) of k -weighted Sn K-edge EXAFS spectra of SnO and STO catalysts. The spectra of the
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samples appear to be quite similar to each other. Peaks at 1-2 Å result from Sn-O neighbors
whereas peaks at 2-4 Å result from Sn-Sn neighbors. The FT-EXAFS data of SnO and STO
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were curve fitted to obtain the structural parameters including coordination number (CN),
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interatomic distances (R), and Debye-Waller factors (σ ). As listed in Table 2, in both samples,
the interatomic distances of Sn-O and Sn-Sn scatterings appeared to be the same. Curve fitting of
the FT-EXAFS revealed that the Sn-O shells were located at 2.05 Å and the two Sn-Sn shells
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were located at about 3.2 and 3.7 Å, confirming that both samples are rutile SnO structures.
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The main difference between the spectra of SnO and STO is in the coordination numbers of Sn-
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Sn neighbor shells. The decreased coordination numbers of the Sn-Sn shells in STO compared to
SnO indicates a smaller crystallite size in STO than SnO , which is consistent with the results
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from the XRD and TEM analyses. The increased coordination number of the Sn-O interaction
for STO was also observed. This increase in Lewis acid sites by sulfation should have an
influence on CH chlorination (see below).
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.2. Methane Chlorination Reaction. Figure 6 shows the temperature-dependent CH4
conversion and product distribution with the blank and three representative catalysts. STO and
o
SZ were the catalysts prepared by sulfation with 3 M H SO and calcination at 500 C. It is noted
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that even in the blank reactor (Figure 6a), CH was consumed with 3.6% of conversion at 300
4
°
C. The conversion almost linearly increased with increasing temperature, and reached 17.5% at
50 °C. This should be due to non-catalytic, thermal radical chlorination. In this condition, the
selectivity of the main product CH Cl exhibited a gradual decrease, from 95.3% at 300 °C to
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9.2% at 350 °C. The amount of CH Cl was rather small with a selectivity ranging from 4.7 to
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8.6% according to the temperature. The product CHCl started to appear at temperature ≥325
3
°C and was less than 2.2% up to 350 °C. CCl was not detected throughout the investigated
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temperature range. The results for SnO (Figure 6b) were quite similar to those obtained from the
2
blank test, indicating pure SnO had negligible catalytic effect on CH chlorination. This is not
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very surprising because this un-sulfated catalyst has a negligible acid site density. On the other
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hand, over the STO catalyst (Figure 6c), which had the largest acid site density among the
catalysts studied here, CH Cl as the target product was mainly produced with a very high
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selectivity of 96.6% even at 350 °C and 22.3% of CH conversion. This is unusual because
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higher (or lower) CH conversion generally correlates with lower (or higher) CH Cl selectivity.
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Furthermore, no CHCl and CCl were detected at all the temperatures studied here. The medium
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acid catalyst level of SZ (Figure 6d) exhibited results similar to reported data. In that study, when
the CH conversion increased from 10 to 24%, the selectivity to CH Cl decreased from 90 to
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0%. Compared to the STO catalyst, although SZ resulted in a somewhat higher conversion of
5.6%, it showed a much lower CH Cl selectivity of 71.8% at 350 °C, giving similar product
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distributions to those obtained radical processes. It means that free radical reactions are still
63
dominant with SZ catalyst. It should be noted that the CH conversion was negligible below
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00 °C in our reaction system. As can be seen in Figure S4, all the blank and the reactions with
the STO and SZ catalysts showed only less than 1% methane conversion at the reaction
temperature of 200 °C and 250 °C.
If the gas phase radical mechanism applied for STO catalysts, we would expect a product
distribution like that of blank test. However, the conversion and selectivity patterns of the STO
catalysts were noticeably different from the blank. This difference suggests that the STO
catalysts may possess distinct catalytic mechanism from the blank. Unlike the radical process, in
an ionic catalytic process, CH Cl adsorbed on the catalytic active site (i.e., Lewis acid site) could
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not be converted to CH Cl and further chlorinated forms, because Cl anion can be more easily
2
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decomposed from CH Cl rather than H. To prove this hypothesis, we also performed CH Cl
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chlorination reaction. In this experiment, chlorination was carried out by injecting CH Cl instead
3
of methane as a reaction feed. We chose 300 and 325 °C as the reaction temperature, where the
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STO catalyst is relatively stable. The CH Cl:Cl ratio was also adjusted to 1:2 for diluted
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conditions of CH Cl. Figure 7 displays the reaction results. In the blank reaction (Figure 7a)
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following the radical mechanism, the conversion of CH Cl was much higher than that of CH as
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expected. That is, the CH Cl conversion was about 59.1% at 300 °C, and the selectivity of
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reached 86.0% and the product contained up to CCl . In this case, the selectivities of CH Cl ,
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CHCl and CCl were 43.6%, 51.7% and 4.7%, respectively. On the other hand, very
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interestingly, it was observed that the conversion of CH Cl was very low in the reaction using
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STO catalyst (Figure 7b). At 300 °C and 325 °C, the CH Cl conversion was only 3.7% and
3
7.4%, respectively, and the product was found to be CH Cl only. Thus, chlorination of CH Cl is
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not preferred on STO catalysts, which may also be evidence of high CH Cl selectivity of STO in
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CH chlorination.
4
We found that the major difference between STO and SZ is in their acidic properties: (1) much
more acid sites of STO than SZ especially in strong acid sites from NH -TPD (Figure 4a), and
3
(2) relatively more Lewis acid sites (lower B/L ratio) with stronger Lewis acidity (band shift to
high energy wavenumber) in STO compared to SZ from pyridine-IR (Figure 4b). These results
suggest that over the strong Lewis acid sites, the non-selective thermal radical chlorination of
CH can be suppressed to lead better selectivity of the target CH Cl.
4
3
The results of the time-on-stream (TOS) performances at 300, 325 and 350 °C are shown in
Figure S5. The conversion and selectivities to the three chloromethane compounds were almost
o
maintained with TOS at lower temperatures, 300 and 325 C. There was only slight increase or
o
decrease. However, at 350 C, the selectivity to CH Cl decreased with TOS but the conversion
3
and the selectivities to CH Cl and CHCl increased. This means the STO catalyst lost the
2
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catalytic property due to some reasons, and thus the free radical reaction occurred. One
possibility is the active site poisoning by coke deposition, but this can be excluded because there
was no coke species in the spent catalyst as described above. Then, another possibility is the
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leaching of active site. To verify this phenomenon, we measured the CH -TPR for the three
4
catalysts. As shown in Figure S6, unlike the other two catalysts, STO was reduced at higher than
o
3
50 C, which may be due to the reduction of SnO to Sn. Then, Sn can leach out as a following
x
equation, Sn + 2Cl → SnCl
(b.p. ~ 114 °C).
4, gas
2
The effects of calcination temperature and sulfate content of STO catalyst on the CH4
chlorination performance were evaluated at reaction temperatures of 300, 325, and 350 °C. As
shown in Figure 8a, STO sulfated with 3 M H SO and calcined at 500 °C (STO-500 °C)
2
4
exhibited the highest conversion with a high CH Cl selectivity in the range of 96-98%. However,
3
calcination at 600 °C resulted in lower sulfur content than 500 °C (0.9 vs 2.6 wt%), and thus
STO-600 °C led to both lower conversion and lower CH Cl selectivity. In addition, with the
3
increase in sulfur content (2.3-2.6 wt%) produced by increasing H SO concentration (1-3 M),
2
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CH conversion gradually increased at all reaction temperatures studied here, while the CH Cl
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selectivity remained almost constant >95% even at 350 °C (Figure 8b). Figure 8c shows the
effect of molar ratio of CH :Cl on conversion and selectivity over STO catalyst. It is well
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known that CH conversion increases and CH Cl selectivity decreases with an increase in the
4
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6,27
relative amount of Cl in the reactant mixture.
Over the STO catalyst, when the CH :Cl ratio
4 2
2
was changed from 1.5:1 to 1:3, CH conversion drastically increased and reached 36% at 350 °C.
4
We should note here that although CH Cl selectivity also decreased, the rate of decrease was
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relatively slow and reached 87% at 350 °C.
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experiments on the STO catalyst were compared with the blank reaction, which is a thermal
radical reaction. The catalyst amount was the same and the flow rate was changed. As shown in
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Figure S7, both the blank and STO catalyst showed that the CH conversion gradually decreased
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due to the decrease in contact time (or residence time) with increasing GHSV. Meanwhile,
CH Cl selectivity showed different trends in the two reactions. In the blank reaction, as expected,
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CH Cl selectivity gradually increased as GHSV increased. This shows a typical characteristic of
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the free radical reaction, where shorter reaction times result in less polychlorinated products. On
the other hand, reactions with STO catalysts maintain very high levels of CH Cl selectivity over
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the investigated temperature range and are slightly affected by the GHSV changes. In particular,
it was observed that the CH Cl selectivity decreased even more in the case of the relatively harsh
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conditions of the reaction temperature of 350 °C and the space velocity of 4000 ml h g . The
decreased selectivity under higher GHSV could be due to the deactivation of the catalyst caused
by contacting the catalyst with a relatively large amount of methane and chlorine per unit time,
in which the reduction of the catalytic function, i.e., the loss of the active site by leaching, could
be accelerated.
From the overall results, we demonstrated that although the free radical reaction cannot be
completely ruled out, these experimental results suggest that the reaction on the surface of the
STO catalyst works predominantly through a different reaction pathway than the radical
mechanism.
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.3. DFT Calculations. To understand the outstanding performance of the STO catalyst in the
methane chlorination, we computationally investigated the interactions between Cl atoms and the
(110) surfaces of SnO , STO, and SZ with DFT calculations. By forming or breaking the Cl-Cl
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bond, two atomistic models were prepared for each surfaces. Considering experimental
evidences, we construct our computational models by assuming that Cl molecules are cleaved
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species like Cl and Cl , on STO and SZ surfaces while they are cleaved homolytically on
SnO surface. Although these models have fundamental limitations as we performed DFT
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calculations on representative structures, they are reasonable as the electrophilic halogenation is
initiated by forming a complex of the super-acidic catalyst and a polarized halogen molecule.28
In addition, we also checked the homolytic dissociation of Cl molecules on STO surface and
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observed that heterolytic cleavage of Cl molecule is favored for the STO catalyst. As shown in
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Figure 9, we found that the STO surface stabilizes dissociated Cl atoms more than SZ and SnO2
surfaces. This computational result suggests that the STO surface can activate more Cl2
molecules in a heterolytic manner compared to other surfaces, and positively charged Cl atoms
on the STO surface can substitute a hydrogen atom in methane molecules through electrophilic
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insertion reaction. On the other hand, these heterolytically dissociated Cl atoms on the STO
have the positive charge of 0.32 as estimated from the Bader charge analysis,47,48 which is lower
than that (0.53) on the SZ surface. In addition, the activation energy of C-H bond in CH is
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inversely proportional to the positive charges on Cl atoms (Figure S8). In other words, the C-H
activation of CH is more difficult on the STO surface. This result may be the reason why SZ
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catalyst showed the higher conversion than STO as discussed in Figure 6.
We also checked the adsorption energy of the Cl molecule on these surfaces. Surface
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structures with adsorbed Cl and CH Cl molecules are shown in the Figure 10. Cl adsorption
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energies were negative for all the surfaces, indicating that those (110) surfaces were stabilized by
an adsorbed Cl molecule. The STO(110) surface showed the most negative adsorption energy of
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0.81 eV, while the SnO (110) surface showed the least negative value of -0.67 eV. A similar
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positive charges on the surface developed by the electron-withdrawing sulfate group. For
understanding the higher selectivity toward the mono-chlorination, we additionally prepared the
surface structures with adsorbed CH Cl molecules. The adsorption energy of CH Cl on the
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STO(110) surface can also be calculated as the most negative (-1.20 eV) compared to those for
SnO (110) and SZ(110) surfaces (-1.03 and -0.82 eV, respectively). In addition, we
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CH Cl (Figure S9). It was observed that CH Cl was detached from the catalyst surface in the
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transition state structure, which means that CH Cl on STO should overcome the high adsorption
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energy to proceed the reaction through the transition state structure. More importantly, in
contrast to C-H activation by Cl radicals in the gas phase (Figures S9a and b), it has been found
that C-H activation becomes more difficult after the mono-chlorination for STO (Figures S9c
and d). Thus, it can be speculated that STO prevents the further chlorination of CH Cl by holding
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CH Cl molecules stronger than other catalysts, which also can be supported by the results of
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CH Cl chlorination experiments (Figure 7).
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.4. Relationship between Conversion and Selectivity. Figure 11 summarizes the
relationship between CH conversion and CH Cl selectivity over the several solid catalysts
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studied here, and reported in some literature.64-72 It is obvious that there appears to be an inverse
linear relationship between the CH conversion and CH Cl selectivity. The SZ and its several
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derivative catalysts display enhanced selectivity to CH Cl compared to the trade-off line of blank
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run or SnO at the same conversion level, but the differences are not very great. Most
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interestingly, the STO catalysts exhibit remarkably enhanced CH Cl selectivity compared to the
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other catalytic results. At 25% CH conversion, the selectivity was almost 90%, which is 25%
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higher than the trade-off line. Despite the large number of disparate approaches for direct CH4
transformation, none of them has been developed into an industrial process. One of the biggest
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problems is the very low selectivity to desired product with reasonably high CH conversion. For
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example, when considering the results of direct CH to CH OH, which can act as a platform
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molecule like CH Cl, the most sophisticated homogeneous and heterogeneous catalysts showed
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below 90% selectivity to CH OH, with only 10% CH conversion. Therefore, the highly
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selective CH chlorination over STO catalyst has the potential to advance the commercialization
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of the direct methane transformation process.
4
. CONCLUSION
In this study, we have demonstrated that STO efficiently catalyzed the chlorination of CH to
4
CH Cl with good conversion and high selectivity compared to other catalyst systems. Based on
3
the experimental and computational results, we speculate that the improved performance of STO
is likely due to the abundant presence of strong Lewis acid sites developed by the interaction of
Sn and surface sulfate groups, thus improving adsorption of Cl and CH Cl on the catalyst
2
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surfaces. It was also computationally shown that heterolytic cleavage of Cl molecule is more
2
favored for the surface of STO and positively charged Cl atoms can activate C-H bond and
promote the electrophilic chlorination. This work on the chlorination of CH over STO suggests
4
an interesting alternative for activating CH to valuable chemicals. We will undertake more
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detailed mechanistic studies in the near future with advanced operando characterization and
theoretical modeling to understand how the selective chlorination of CH occurs on the surface
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of STO catalysts.
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AUTHOR INFORMATION
Corresponding Author
*
E-mail: hjchae@krict.re.kr (H.-J. Chae)
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Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript. †These authors contributed equally.
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the ACS Publication website at DOI:.
Elemental analysis of spent catalyst. Summary of catalyst performance. Gas chromatographs.
TGA curves for the fresh and spent catalysts. FT-IR spectra of pyridine adsorption. CH4
chlorination reaction results at low temperature. Time on stream in CH chlorination. CH -TPR
4
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curves. GHSV effect in CH chlorination. Effect of the positive charged Cl atom to the C-H
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activation energy. Transition state energies for the C-H activation of CH and CH Cl.
4
3
ACKNOWLEDGMENT
This work was supported by the C1 Gas Refinery Program through a grant from the National
Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT
(2016M3D3A1A01913251).
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